.eta.2-Dihydrogen on the brink of homolytic cleavage: trans-[Os(H.cntdot..cntdot..cntdot.H)H(PEt2CH2CH2PEt2)2]+ has spectroscopic and chemical properties between those of the isoelectronic complexes trans-[OsH(PPh2CH2CH2PPh2)2(.eta.2-H2)]+ and ReH3(PPh2CH2CH2PPh2)2

Abstract

The new, octahedral complex trans-[OsH(dppe)2(η²-H₂)]⁺, 10s, dppe = PPh₂CH₂CH₂PPh₂, prepared by addition of H⁺ to cis-OsH₂(dppe)₂, has a rapidly spinning dihydrogen ligand with an unusually long (0.99 Å) H–H distance according to the T₁ NMR method. Additional evidence for this is a short T₁ value for the H₂ ligand in the complex trans-[OsH-(dppe-d₂₀)₂(η² H₂)]⁺ and a large ¹J(H,D) value (25.5 Hz) for trans-[OsH(dppe)₂(η² -HD)]⁺, 1Os-d₁. The known complex trans-[Os(H⋯H)H(depe)2]⁺, 2Os, depe = PEt₂CH₂CH₂PEt₂, has properties (δH, J(H,D), T₁⁻¹) between those of 1Os and ReH₃(dppe)₂, 3Re, a true pentagonal, bipyramidal trihydride with r(H⋯H) ~ 2 Å; e.g., the J(H⋯D) coupling for trans-[Os(H⋯D)D(depe)₂]⁺, 2Os-d², is the unusual value of ~11.5 Hz, whereas for 3Re-d₂ it is 0 Hz, as expected. Significantly the 11.5-Hz value varies by ±1 Hz depending on the solvent and temperature. VT NMR spectra of 2Os-d₂ conclusively show that J(H⋯D) is not lost but is averaged by intramolecular H/D atom exchange; at 325 K, J_av(H,D) = 3.8 Hz. There are three possible answess to the question as to whether there is homolytic cleavage of H₂ in 2Os: (1) no H–H cleavage so that 2Os, like 1Os, has a stretched, rapidly spinning dihydrogen ligand with r(H–H) = 1.2 Å; (2) complete H–H bond breaking to give an unusual trihydride with two closely spaced hydrides, 1.4–1.5 Å apart; and (3) a rapidly equilibrating mixture (ΔG‡ ≤ 9 kcal mol⁻¹) of Os(H)(η²-H₂) and Os(H)₃ tautomers. The rapid equilibrium model (3) with the Os(H)₃ tautomer in greater abundance fits all available data for 2Os and other complexes of Os with small J(H⋯D) couplings. The equilibrium shifts toward the dihydrogen form with an increase in temperature or on going from acetone-d₆ to CD₂Cl₂ as signaled by changes in δ(H₂), T₁(H₂), and J(H,D) values. An alternative answer (2) cannot be ruled out since closely spaced hydrides might move closer together with these changes in conditions and produce the same spectral changes. Also included are X-ray diffraction structural data for the three complexes, their infrared, chemical, and electrochemical properties as well as differences in NMR properties: rates of intramolecular H-atom exchange, chemical shifts, and coupling constants of the isotopomers 1Os-d₁, 2Os-d₁, 2Os-d₂, and 3Re-d₂. 1Os retains the H–H bond because it has lower energy 5d electrons than 2Os or 3Re, and the dppe ligands are small enough to allow an undistorted octahedral coordination geometry. 3Re gives shorter T₁ times for hydride and phosphorus nuclei than expected solely from dipolar relaxation by neighboring protons.